The effectiveness–NTU method is convenient when inlet temperatures and UA are known but outlet temperatures are not. The heat-capacity rates are C = ṁcₚ; the smaller one is Cmin. The maximum thermodynamically possible duty is:
Qmax = Cmin(Th,in − Tc,in)
Actual duty is Q = εQmax, where effectiveness ε depends on NTU, heat-capacity ratio and flow arrangement.
When all four terminal temperatures are known, the logarithmic mean temperature difference is:
LMTD = (ΔT₁ − ΔT₂) / ln(ΔT₁/ΔT₂)
Counterflow and parallel-flow exchangers use this directly. Multipass shell-and-tube exchangers require a correction factor F; this calculator reports the standard one-shell-pass screening expression. Crossflow duty is calculated by the ε–NTU method, and a single LMTD correction factor is not reported by this simplified model.
Shell-and-Tube Screening mode estimates tube-side convection with a laminar/developing-flow expression below Reynolds number 2300, a blended transition between 2300 and 4000, and the Gnielinski correlation for turbulent flow. Shell-side convection uses a simplified Kern-type equivalent-diameter treatment. Pressure drops include straight-flow friction and approximate return/baffle losses.
Important: shell-side leakage, bypass streams, entrance effects, vibration, unequal pass distribution and detailed nozzle losses are not modelled. The arrangement is fixed to one shell pass with an even number of tube passes.
Can the cold outlet become hotter than the hot outlet?
Yes, in a counterflow exchanger when the hot stream has the larger heat-capacity rate. The cold outlet must still remain below the hot inlet temperature. Parallel flow cannot produce this terminal-temperature crossover.
Why can two exchangers with the same UA give different duties?
Duty also depends on inlet temperatures, stream heat-capacity rates and flow arrangement. Counterflow normally gives the highest effectiveness for the same NTU and capacity ratio.
What does fouling resistance do?
Fouling adds thermal resistance between the fluids. In geometry mode it lowers the calculated overall U-value and UA, reducing duty and changing outlet temperatures.
Why are pressure-drop results only approximate?
The model uses simplified tube-friction and Kern-type shell-side screening correlations. It does not resolve leakage, bypass flow, nozzle losses, detailed baffle geometry, vibration limits or pass-to-pass maldistribution.
When should I use LMTD instead of effectiveness–NTU?
LMTD is most convenient when all terminal temperatures are known. Effectiveness–NTU is usually easier for rating a known UA when the outlet temperatures are unknown.
Does geometry mode represent a vendor design?
No. It is a preliminary educational model with the hot fluid assigned to the tube side and the cold fluid to the shell side. Final design requires validated properties, TEMA mechanical checks and specialist rating software or vendor calculations.
EngineerHub.org